BRINTON: POPULATION BIOLOGY OF EUPHAUSIA PACIFICA 



expected: onset of juvenile phase, onset of gonad 

 development, and time of maximum gamete 

 production. 



Biomass on body-length distribution was most 

 even during 1953 and 1954 (Figure 16a, b). 

 Recruitment in May and September 1953 led to the 

 7-mm peak of that year, and the September cohort 

 was the main contributor to the 10-12 mm peak. 

 The 1954 crest at 10-12 mm stemmed mainly from 

 October and December sampling of the June 1954 

 cohort. 



In 1955 and 1956, 3-4 mm larvae were reduced in 

 average biomass compared with 1953 and 1954 

 while biomass of 9-12 mm adolescents was 'two 

 times greater. The November 1955 stock of 11-12 

 mm stages (5507 cohort) was mainly responsible 

 for the 1955 biomass peaks. The November- 

 December 1956 stock of 9-11 mm stages (5609-10 

 cohort) provided much of the 1956 peak. 



Large 18-20 mm adults showed their greatest 

 biomass in 1956 following the strong upwelling 

 year 1955, and lowest in 1955 following least 

 productive year 1954. 



Monthly changes in biomass are traced for each 

 of three conspicuous sizes (Figure 17). Small (7 mm) 

 juvenile bulk is greatest within May-July follow- 

 ing spring recruitment. Other high values for the 

 7-mm size are not consistent seasonally, occurring 

 during October-March. 



Adolescents (10 mm), considered representative 

 of the 9-12 mm juvenile-adult phase change, tend 

 to be at greatest volume during August-January 

 (when the smallest spawners, 10.6-12.5 mm, were 

 also observed to peak, Figure 4d). Increased sur- 

 vivorship and slowed growth during fall-winter 

 maturation of spring cohorts, discussed above, are 

 considered responsible. 



Subsequent February-March peaking of 

 biomass at 15-mm size occurs as egg development 

 accelerates. (This is preliminary to the appearance 

 of the large >16.5 mm spawners during April- 

 June, Figure 4d.) 



A close relationship is evident (Figure 17) 

 between biomass of each of the three sizes and 

 their percent of the total E. pacifica biomass. This 

 indicates that a given month's increase in biomass 

 of the 7-mm size (or of the 10-mm or 15-mm size) is 

 not accompanied by proportionate increase in the 

 composite biomass of all other sizes. Therefore, the 

 periodic peaks in biomass shown in Figure 5b 

 should be largely due to peaks at these or very 

 similar sizes, which was indeed the case. 



7 mm SIZE 





15mm SIZE 



01 02 03 04 05 06 07 

 MONTH 



10 II 



12 



< 

 O 



Q 



I 



Q 

 O 



E 

 O 



o 

 o 



a. 



E 



< 



O 



CQ 



Figure 17. -Annual length-biomass modes of Eupkausia 

 pacifica analyzed by months. The 7-mm size peaks heavily in 

 June-July (May-June in 1956), with other peaks in March and 

 November-January; the 10-mm size peaks September-January; 

 and 15-mm size peaks February or March. 



There are variations from this relationship: 1) 

 moderate increase in biomass of 10-mm size dur- 

 ing August-December 1954 caused a dispropor- 

 tionately large percent-increase in it-an effect of 

 the single large 1954 cohort (5406-07) developing 



753 



